rhbz#1043551: sw: avoid division-by-0 in Text Grid painting code
rhbz#1043551: sw: avoid division-by-0 in Text Grid painting code

Possible to trigger with a document containing:
    style:layout-grid-base-height="0cm"

Change-Id: Id3bd1f29157b39e8a577be0b87b86236dbe5a50c

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core